1. Field of the Invention
The present invention relates to a photoelectric encoder. In particular, the invention relates to improvements in a photoelectric encoder that has a telecentric optical system wherein a lens and an aperture are inserted between a main scale and a light receiving element.
2. Description of the Related Art
As described in Japanese Patent Laid-Open Publication No. 2004-264295 and as shown in FIG. 1, a photoelectric encoder is designed in which a lens optical system (telecentric optical system) 40, comprising a lens 42 and an aperture 44 that functions as a telecentric optical diaphragm, is inserted between a main scale 20 and a light receiving element array 34 constituting a light receiving unit 30, for example, and as shown in FIG. 2 this lens optical system can set the magnification by adjusting the distances a and b between the lens 42 and the scale 21 of the main scale 20 and between the lens 42 and the light receiving element 35 on the light receiving element array 34, respectively. In FIG. 1, the reference numeral 10 denotes a light source and the reference symbol f denotes a focal length of the lens 42.
In the photoelectric encoder that uses this telecentric optical system 40, an image on the main scale 20 is made pass through the lens optical system (42, 44) and is projected onto the light receiving element array 34. Here, by positioning the aperture 44 at the focal position of the lens 42, even when the distance (gap) between the main scale 20 and the lens 42 changes, fluctuations in the magnification of the image formed on the light receiving element array 34 can be controlled if the positional relationship between the lens 42, the aperture 44, and the light receiving element array 34 does not change.
In Japanese Patent Laid-Open Publication No. Hei 10-82611, it is described that, utilizing the Scheimpflug rule, light is incident on a wafer from a lateral narrow slit provided in a reticle projection optical system.
When constituting a photoelectric encoder of a reflection type by using a telecentric optical system as described in Japanese Patent Laid-Open Publication No. 2004-264295, however, if a half mirror 46, at the center of which an aperture 44 is formed as shown in FIG. 3, is inserted, the amount of the light that passes through the half mirror 46 twice and reaches a light receiving element 34 is reduced by (½)×(½)=(¼). Therefore, to offset this reduction, supply current to a light source 10 needs to be increased fourfold. In addition, since the optical axis is perpendicular to the surface plane of the main scale 20, it is impossible to make the optical system smaller. In FIG. 3, the reference numeral 12 denotes a collimator lens that collimates the light emitted from the light source 10.
When the optical axis is inclined as shown in FIG. 4, the air gaps between the main scale 20 and the lens 42 at the central and edge regions differ from each other as shown in FIG. 5. Therefore, if focus is obtained at the center, for example, blurring occurs at the upper and lower regions, so that a uniform contrast cannot be ensured over the entire light receiving surface, causing the problem of a reduction in signal detection efficiency.
Until now, the feasibility of applying the Scheimpflug rule to a photoelectric encoder using a telecentric optical system has not been considered.